Scheduling for non-real-time services in orthogonal frequency division multiplex (OFDM) systems

ABSTRACT

Sub-carriers are allocated among the plurality of users in the OFDM system based on normalized channel gain values. Then, data bits are allocated for transmission on the allocated sub-carriers based on incremental transmission cost values. The channel gain values and the incremental transmission cost values are computed using a calculated fairness factor for each user in the system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.60/553,845, filed on Mar. 17, 2004, which is incorporated by referenceas if fully set forth.

FIELD OF INVENTION

The present invention relates to OFDM systems. More particularly, thepresent invention is a method and apparatus for scheduling non-real-timeservices in multi-user OFDM systems.

BACKGROUND

Future wireless communication networks will provide broadband servicessuch as wireless Internet access to subscribers. Those broadbandservices require reliable and high-rate communications over hostilemobile environments with limited spectrum and intersymbol interference(ISI) caused by multipath fading. Orthogonal frequency divisionmultiplex (OFDM) is one of the most promising solutions to address theISI problem. In fact, OFDM has been chosen for European digital audioand video broadcasting, and wireless local area network (WLAN)standards, such as, for example, 802.11a.

In single user OFDM systems, techniques such as the water-fillingapproach may be utilized to determine an appropriate sub-carrier and bitallocation solution that minimizes total transmit power. In multi-userOFDM systems, however, such determinations are much more difficult.Sub-carriers in a multi-user system, for example, may be desirable tomore than one user at the same time. Such variables add a great deal ofcomplexity to the determining of optimal power allocation schemes, i.e.,the scheduling of non-real-time (NRT) services.

Existing approaches directed at scheduling NRT services in multi-usersystems utilize a non-linear optimization approach, such as, forexample, the Lagrangian heuristic procedure with relaxation. Thesenon-linear approaches, however, require intensive computations and areincapable of yielding optimal solutions. At most, such approaches yieldonly lower and upper bounds.

None of the existing approaches, including the non-linear approachesdiscussed above, consider ‘fairness’ between users in scheduling NRTservices. Fairness, as further described below, is an indication of thequality of services (QoS) experienced by one user in the system versusthe QoS expected, or the QoS experienced by other users in the system.By not considering fairness, the QoS experienced by certain users willbe far superior to the QoS experienced by other users.

To illustrate, consider FIG. 1. In a multi-user OFDM network 100, basestation 110 is servicing wireless transmit/receive units (WTRUs) 102,104, and 106. Each WTRU has a respective channel gain G_(k,n) on aparticular sub-carrier, where k represents the sub-carrier and nrepresents the user, or in this case, the WTRU. As illustrated in theFigure, WTRU 102 has a channel gain on sub-carrier k equivalent toG_(k,102). Similarly, WTRUs 104 and 106 have channel gain values ofG_(k,104) and G_(k,106), respectively. If, for example, WTRU 102 has ahigher channel gain value G_(k,102) than the other WTRUs 104, 106,sub-carrier k will be allocated to WTRU 102. As long as WTRU 102continues to have a channel gain value G_(k,102) superior to that ofWTRUs 104 and 106, it will continue to occupy sub-carrier k, and thuscontinue to experience superior system performance. In the mean time,WTRUs 104 and 106 continue to suffer sub-par performance at the expenseof WTRU 102. If fairness between users were utilized in system 100,however, WTRUs 102, 104 and 106 would all experience comparable systemperformance, in spite of their individual channel gain values.

Accordingly, it is desirable to have a method and apparatus forscheduling NRT services in a multi-user wireless OFDM system thatconsiders and maintains fairness between users and does not requireintense computations.

SUMMARY

The present invention relates to a method and apparatus for schedulingnon-real-time (NRT) services in a wireless, multi-user orthogonalfrequency division multiplexing (OFDM) communication system. First,sub-carriers are allocated among the plurality of users in the OFDMsystem based on normalized channel gain values. Then, data bits areallocated for transmission on the allocated sub-carriers based onincremental transmission cost values. The channel gain values and theincremental transmission cost values are computed using a calculatedfairness factor for each user in the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a wireless, multi-user orthogonal frequency divisionmultiplexing (OFDM) system;

FIG. 2 is a sub-carrier allocation device configured to operate in awireless, multi-user OFDM system;

FIG. 3 is a bit allocation device configured to operate in a wireless,multi-user OFDM system; and

FIG. 4 is a flow diagram for scheduling non-real-time services in awireless, multi-user OFDM system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone (without the other features andelements of the preferred embodiments) or in various combinations withor without other features and elements of the present invention.

Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, mobile station, fixed or mobile subscriberunit, pager, or any other type of device capable of operating in awireless environment. When referred to hereafter, a base stationincludes but is not limited to a Node-B, site controller, access pointor any other type of interfacing device in a wireless environment.

In a wireless Orthogonal Frequency Division Multiplexing (OFDM) system,non-real time (NRT) services for multiple users are scheduled such thatthe system's resources are efficiently utilized while maintaining aconsistent quality of service (QoS) level amongst the users.Sub-carriers are first individually allocated amongst the users,followed by a bit-by-bit allocation of data bits for transmission on thesub-carriers. The sub-carrier and bit allocation functions of thepresent embodiment consider ‘fairness’ to the users in performing theirrespective allocation duties.

Fairness, as described herein, is a factor unique to each user in theOFDM system. This fairness factor indicates the quality of servicesbeing provided to each user as compared to the QoS expected or the QoSbeing experienced by other users in the system. In determining theusers' fairness factors, any quality metric deemed appropriate may beutilized. Data transmission rate, bit error rate, and the like, forexample, may be utilized to determine a fairness factor for each user inthe system.

In allocating the system's sub-carriers amongst the users, a channelgain value for each user on each sub-carrier is determined. Next, thesechannel gain values are normalized using each user's fairness factor.Each sub-carrier is then allocated to the user with the highestnormalized channel gain value on the respective sub-carrier. It shouldbe noted that this method of allocation allocates each sub-carrier to atmost one user (i.e., the one user with the highest normalized channelgain on that sub-carrier), thus avoiding interference that may arisefrom multiple users utilizing a single sub-carrier at the same time. Onthe other hand, multiple sub-carriers may be allocated to a single user.In fact, those users with the lowest fairness factors are more likely toreceive multiple sub-carriers, so as to compensate for their inferiorQoS levels.

Once the sub-carriers are allocated to the users in the OFDM system, anormalized water-filling algorithm is utilized to allocate data bits ona bit-by-bit basis for transmission on the sub-carriers. To allocate adata bit for transmission, an incremental ‘cost’ value for transmittingone bit of data on each sub-carrier is calculated. This incremental costvalue is the additional power required to transmit one bit of a user'sdata on one of the sub-carrier(s) allocated to the user, multiplied bythe user's fairness factor. Once an incremental cost value is calculatedfor each sub-carrier, the sub-carrier yielding the lowest incrementalcost value, while not exceeding a maximum allowable transmit powerconstraint, is selected to transmit this data bit. It should be notedthat this data bit is of the user to whom the selected sub-carrier isallocated.

This allocation process is then repeated until all bits are allocatedfor transmission, or until the maximum allowable transmit powerconstraint is reached, whichever occurs first. Utilizing this normalizedwater-filling bit allocation technique not only maximizes the amount ofdata that is transmitted, but it does so ‘fairly’ and without exceedingthe maximum allowable transmit power.

Referring now to FIG. 2, a sub-carrier allocation device 201 configuredto operate in a servicing base station is shown operating in a wireless,multi-user OFDM system 200. The present device 201 comprises acalculating instantaneous channel gain values device 204; a calculatinga fairness factor for each user in the system 200 device 206; anormalizing the calculated channel gain values using the fairnessfactors device 208; and an allocating each sub-carrier 202 ₁-202 _(k) inthe system 200 to the user with the highest normalized gain values onthat sub-carrier device 210. For purposes of this illustration, it isassumed that there are N users and K sub-carriers in the multi-user OFDMsystem 200. Utilizing the gain calculation device 204, the sub-carrierallocation device 201 calculates a channel gain value G_(k,n) for eachuser on each sub-carrier 202 ₁-202 _(k) in the system 200, where G_(k,n)represents the channel gain value between the n^(th) user and aservicing base station on the k^(th) sub-carrier. A fairness factorf_(n) for each user is then determined utilizing the fairnesscalculation device 206, where f_(n) denotes the fairness factor for then^(th) user.

As previously described, any quality metric deemed appropriate may beutilized in determining fairness. Regardless of the metric utilized,however, the fairness factors should be indicative of: 1) a user'sresource usage as compared to the resource usage of other users in theOFDM system 200 during a given time frame; or 2) actual systemperformance experienced by a user versus the system performance expectedand/or required by the user. Performance may indicate a user'sexperienced throughput, delay, data rate, etc. An example definition offairness as described herein is given by Equation 1 below:$\begin{matrix}{{{fairness}(n)} = \frac{R_{ave}(n)}{R(n)}} & {{Equation}\quad 1}\end{matrix}$where R(n) may denote, for example, the data rate configured by thesystem 200 for user n, i.e., the data rate expected by user n, andR_(ave)(n) is the average data rate actually experienced by user n overa specified period of time. According to Equation 1, if user nexperiences a lower data rate than expected, his fairness factor will berelatively low. Alternatively, experiencing a data rate at or above whatis expected will result in a relatively high fairness factor for user n.It should be noted that Equation 1 is but one example formula fordetermining fairness. This formula may be manipulated to accommodateother performance metrics, such as delay or bit error rate (BER), whosedesirability increases as their values decrease. If delay, for example,were selected as the performance metric by which fairness is to bedetermined, Equation 1 might be modified such that fairness(n) equalsD(n) divided by D_(ave)(n), where D(n) denotes the delay configured bythe system 200 for user n, and D_(ave)(n) denotes the average delayactually experienced by user n over a specified period of time.Accordingly, if a user n were experiencing a higher delay than expected,his fairness would be relatively low, while experiencing a lower delaythan expected would result in a higher fairness factor.

Referring back to FIG. 2, once a fairness factor f(n) is calculated foreach user n in the OFDM system 200, each user's calculated channel gainvalues (G_(k,n)) are normalized according to the user's calculatedfairness factor f(n) utilizing a normalizing function in the normalizingdevice 208. The resulting normalized channel gain values (NG_(k,n)) 208′may be expressed according to Equation 2 below: $\begin{matrix}{{NG}_{k,n} = {\frac{G_{k,n}}{{fairness}(n)}.}} & {{Equation}\quad 2}\end{matrix}$

The allocation device 210 then allocates each sub-carrier k to the usern with the highest normalized channel gain value NG_(k,n) on thatsub-carrier. As shown in the Figure, sub-carriers k₁ and k₂ (202 ₁, 202₂) are allocated to user 2, thus it follows that the normalized gainvalues NG_(1,2,) NG_(2,2) for user 2 on sub-carriers k₁ and k₂ (202 ₁,202 ₂) are higher than the normalized gain values of any other user onsub-carriers k₁ and k₂ (202 ₁, 202 ₂). Similarly, since sub-carrierk_(k) (202 _(k)) is allocated to user 1, user 1 has a higher NG_(k,1)value on sub-carrier k₁ (202 _(k)) than any other user.

Once all of the sub-carriers 202 ₁-202 _(k) have been allocated amongstthe users in the system 200, the number of bits that will be transmittedon each sub-carrier 202 ₁-202 _(k) is determined.

Referring now to FIG. 3, a bit allocation device 301 configured tooperate in a servicing base station according to a normalizedwater-filling algorithm is shown operating in a wireless, multi-userOFDM system 300. The bit allocation device 301 has an initializingdevice 302; an incremental cost calculating device 304; a bit allocatingdevice 306; and an allocated bit and total transmit power updatingdevice 308.

For purposes of this Figure, let f_(n)(r) denote a required receivedpower when r bits of a user n are transmitted on a sub-carrier k.Further, let BER_(n) denote a required bit error rate (BER) of a user n.If the system 300 utilizes M-ary QAM, for example, then the requiredpower to transmit r bits per symbol in OFDM system 300 may be expressedaccording to Equation 3 below: $\begin{matrix}{{{f_{n}(r)} = {\frac{N_{0}}{3} \cdot \left\lbrack {Q^{- 1}\left( \frac{{BER}_{n}}{4} \right)} \right\rbrack^{2} \cdot \left( {2^{r} - 1} \right)}},} & {{Equation}\quad 3}\end{matrix}$where N₀ is background noise at each sub-carrier. If r_(k)(n) isutilized to represent the number of bits of an n^(th) user allocated toa k^(th) sub-carrier, in order to maintain a desired quality of service(QoS) level, the allocated transmit power on the k^(th) sub-carrier maybe expressed as in Equation 4 below: $\begin{matrix}{{{P_{k}(n)} = \frac{f_{n}\left( {r_{k}(n)} \right)}{G_{k,n}^{2}}};} & {{Equation}\quad 4}\end{matrix}$where G_(k,n) is the channel gain between a user n and a base station ona k^(th) sub-carrier. It should be understood that Equations 3 and 4 aremerely examples of equations for estimating transmit-power. Otherequations that adequately estimate transmit-power may be utilizedwithout departing from the scope of the present embodiment.

Referring back to FIG. 3, the initialization device 302 initializes thenumber of bits allocated to each user n on a k^(th) sub-carrierr_(k)(n), and the total allocated transmit power P_(total), to zero. Thetotal allocated transmit power P_(total) is defined as the sum of alltransmit powers on all sub-carriers, i.e., the sum of all P_(k)(n) forall k sub-carriers. In a real system, such as OFDM system 300, P_(total)may not exceed a maximum allowable transmit power constraint P_(max)specified for the servicing base station.

Once r_(k)(n) and P_(total) are initialized to zero, the incrementalcost calculating device 304 calculates an incremental cost ΔCost_(k)(n)required to transmit one additional bit 305′ of each user n's data 305on each sub-carrier carrier k allocated to said user n. The incrementalcost value is the additional transmission power required to transmit onebit of user n's data on a sub-carrier k allocated to user n multipliedby user n's fairness factor, as expressed below in Equation 5:ΔCost_(k)(n)=ΔP _(k)(n) fairness(n);   Equation 5where ΔP_(k)(n) is the additional transmission power required totransmit this one bit, which may further be expressed according toEquation 6: $\begin{matrix}{\frac{{f_{n}\left( {{r_{k}(n)} + 1} \right)} - {f_{n}\left( {r_{k}(n)} \right)}}{G_{k,n}^{2}}.} & {{Equation}\quad 6}\end{matrix}$Combining Equations 5 and 6, the incremental cost to transmit oneadditional bit of user n's data on a sub-carrier k allocated to user nmay be expressed as in Equation 7: $\begin{matrix}{{\Delta\quad{{Cost}_{k}(n)}} = {\left( \frac{{f_{n}\left( {{r_{k}(n)} + 1} \right)} - {f_{n}\left( {r_{k}(n)} \right)}}{G_{k,n}^{2}} \right) \cdot {{{fairness}(n)}.}}} & {{Equation}\quad 7}\end{matrix}$The sub-carrier k with the lowest incremental transmission costΔCost_(k)(n) that does not exceed the maximum allowable transmissionpower constraint P_(max), i.e., P_(total)+ΔP_(k)(n)≦P_(max), is selectedto transmit one additional bit of data 305′ by the bit allocationdevice. 306. This additional bit of data belongs to the user n to whichthe selected sub-carrier k was previously allocated. If such asub-carrier k is identified, the updating device 308 updates the numberof bits allocated to the selected sub-carrier by one and increases thetotal allocated transmit power P_(total) by the ΔP_(k)(n) required totransmit this one bit 305′. The bit allocation device 301 then processesthe next bit of data in a similar manner, beginning with the incrementalcost calculating device 304 until either all bits are allocated, oruntil P_(max) is reached, whichever occurs first.

If, however, such a sub-carrier is not identified, i.e., allocating anadditional bit to the sub-carrier k with the lowest transmission costcauses P_(total) to exceed P_(max), (i.e., the total allocated transmitpower exceeds the maximum allowable transmission power constraint), nomore bits are allocated due to lack of transmission power.

A flow diagram for scheduling of non-real time services, i.e.,scheduling transmission of data to multiple users in an OFDM system 400,is shown in FIG. 4. In a multi-user wireless OFDM system 400 having Nusers and K sub-carriers, each. sub-carrier k is allocated to a user n(step 402). A channel gain value G_(k,n) is calculated for each user non each sub-carrier k (step 401). A fairness factor f(n) for each userin the OFDM system 400 is then determined (step 403). Utilizing thefairness factors of step 403, each user's channel gain values of step401 are normalized to yield a normalized channel gain value NG_(k,n) foreach user n on each sub-carrier k (step 405). Each sub-carrier k is thenallocated to the user n with the highest NG_(k,n) value on thatsub-carrier k (step 407).

Once all the sub-carriers have been allocated to the users (step 402),data bits are allocated for transmission on the sub-carriers (step 404).A maximum allowable transmit power constraint P_(max) is determined(step 409). Then, a total allocated transmit power value P_(total), anda total number of bits allocated to each n^(th) user on a k^(th)sub-carrier r_(k)(n) are initialized to zero (step 411). An incrementaltransmit cost value ΔCost_(k)(n) for transmitting an additional bit ofeach n user's data on each sub-carrier k allocated to said user n iscalculated (step 413). Each n user's calculated incremental costvalue(s) (from step 413) accounts for each n user's fairness factor(from step 403). The sub-carrier k with the lowest incrementaltransmission cost ΔCost_(k)(n) that does not exceed P_(max) (from step409) is selected to transmit one additional bit of data (step 415). Thisadditional bit of data belongs to the user n to which the selectedsub-carrier k was previously allocated (step 407). The total allocatedtransmit power P_(total) and the total number of bits allocated to then^(th) user on a k^(th) sub-carrier r_(k)(n) are then updatedaccordingly (step 417). If P_(total) of step 10 is less than or equal toP_(max) (step 419), steps 413-417 are repeated until all bits in thesystem 400 are allocated (step 421) or until P_(total) exceeds P_(max).

Although the elements in the Figures are illustrated as separateelements, these elements may be implemented on a single integratedcircuit (IC), such as an application specific integrated circuit (ASIC),multiple ICs, discrete components, or a combination of discretecomponents and IC(s). Although the features and elements of the presentinvention are described in the preferred embodiments in particularcombinations, each feature or element can be used alone without theother features and elements of the preferred embodiments or in variouscombinations with or without other features and elements of the presentinvention. Furthermore, the present invention may be implemented in anytype of wireless communication system.

1. In a wireless orthogonal frequency division multiplexing (OFDM)communication system, a method of scheduling non-real time datatransmissions to a plurality of users, the method comprising: (a)allocating sub-carriers among the plurality of users based on normalizedchannel gain values; and (b) allocating bits of data for transmission onthe allocated sub-carriers based on incremental transmission costvalues.
 2. The method of claim 1 wherein the sub-carrier allocation step(a) further comprises: (a1) determining a channel gain value for eachuser on each sub-carrier; (a2) calculating a fairness value for eachuser in the OFDM system; (a3) normalizing each user's determined channelgain values from step (a1) with said user's calculated fairness valuefrom step (a2); and (a4) allocating each sub-carrier to the one userwith the highest normalized channel gain value on each said sub-carrier.3. The method of claim 2 wherein the bit allocation step (b) furthercomprises: (b1) determining a maximum allowable transmit power; (b2)initializing a total allocated transmit power value and a total numberof bits allocated for transmission value to zero; (b3) calculating anincremental cost value for transmitting one bit of data to each user oneach of the sub-carrier(s) allocated to said users; (b4) allocating tothe sub-carrier with the lowest incremental cost value one bit of datafor transmission to the user to whom said sub-carrier was allocated;(b5) increasing the total allocated transmit power value by a transmitpower value required to transmit the allocated bit; (b6) increasing thetotal number of bits allocated for transmission value by one; and if thetotal allocated transmit power value of step (b5) is less than themaximum allowable transmit power of step (b1): (b7) repeating steps (b3)through (b6).
 4. The method of claim 3 wherein the incremental costvalue is the additional power required to transmit one bit of a user'sdata on one of the sub-carrier(s) allocated to said user, multiplied bythe user's fairness factor
 5. The method of claim 2 wherein eachfairness value is a measure of system resources consumed by one userversus system resources consumed by the other users during apredetermined period of time;
 6. The method of claim 2 wherein eachfairness value is a ratio of a system performance metric experienced bya user over a predetermined period of time versus a system performancemetric expected by the users over the period of time.
 7. The method ofclaim 6 wherein the system performance metric is data throughput rate.8. The method of claim 6 wherein the system performance metric is biterror rate (BER) or delay.
 9. A base station configured to schedulenon-real time data transmissions to a plurality of users in a wirelessOFDM system, the base station comprising: means capable of allocatingsub-carriers among the plurality of users based on normalized channelgain values; and means capable of allocating bits of data fortransmission on the allocated sub-carriers based on incrementaltransmission cost values.
 10. The base station of claim 9 furthercomprising: means capable of determining a channel gain value for eachuser on each sub-carrier; means capable of calculating a fairness valuefor each user in the OFDM system; means capable of normalizing eachuser's determined channel gain values with said user's calculatedfairness value; and means capable of allocating each sub-carrier to theone user with the highest normalized channel gain value on each saidsub-carrier.
 11. The base station of claim 10 further comprising: meanscapable of determining a maximum allowable transmit power; means capableof initializing a total allocated transmit power value and a totalnumber of bits allocated for transmission value to zero; means capableof calculating an incremental cost value for transmitting one bit ofdata to each user on each of the sub-carrier(s) allocated to said users;means capable of allocating to the sub-carrier with the lowestincremental cost value one bit of data for transmission to the user towhom said sub-carrier was allocated; means capable of increasing thetotal allocated transmit power value by a transmit power value requiredto transmit the allocated bit; and means capable of increasing the totalnumber of bits allocated for transmission value by one.